The invention relates generally to web guide apparatus that correct lateral displacements of a travelling web. In particular, the present invention provides a web guide apparatus for use in a high speed printing system that not only corrects large positional offsets, but also corrects for high frequency oscillations in the lateral displacement of the web.
In a high speed printing system, for example, a multi-color printing press, several printing operations are generally performed on a continuous travelling paper web. Proper registration of the web in relation to the various printing operations is required in order to produce a satisfactory end product. On occasion, however, the web may become laterally misaligned, necessitating the use of a web guide apparatus that senses the lateral position of the web and automatically repositions the web.
Various types of web guide apparatus have been employed to correct for lateral displacement of the web. One type in particular employs a movable carriage that is attached to a frame support. Parallel idler rollers are connected to the movable carriage, which in turn is pivoted about a fixed point by a motor in order to guide the web. A sensor is employed to detect the lateral displacement of the web.
While the aforementioned carriage-type web guides perform satisfactorily to correct for slowly varying offsets in the web, they suffer from a serious drawback, namely, the inability to correct for high frequency oscillations in the travelling web. It has been found that high frequency oscillations, e.g., up to on the order of approximately 8 Hz, hereinafter referred to as "web weave", cause smearing to occur in the printing operation. Smearing is especially acute in the first printing operation of a multi-color printing press, as will be explained. The available carriage type web guides are unable to cope with the problem of web weave.
The problem of web weave becomes even more critical as overall system speed capability increases. Thus, it is imperative that current web guides inability to correct for web weave be overcome to avoid web weave becoming a limiting factor in system printing speed.